The invention relates to a control system for a continuously variable transmission or CVT.
Such control system and CVT are generally known, for example from the European patent publication EP-A-0.451.887. The CVT comprises a first rotatable pulley and a second rotatable pulley both provided with two pulley discs and with a piston/cylinder assembly for urging the pulley discs towards each other under the influence of a hydraulic cylinder pressure in the piston cylinder assembly. The pulley discs exert a clamping force on a drive belt, which is located between the pulley discs, such that torque transmission between the pulleys and the drive belt is enabled. The clamping force and the coefficient of friction between drive belt and pulley discs determine the maximum torque at which the torque transmission occurs virtually without mutual movement of drive belt and pulley discs, i.e. without belt slip. The ratio between the cylinder pressure in the piston/cylinder assembly of the first pulley and that in the piston/cylinder assembly of the second pulley determines the transmission ratio, i.e. the ratio between a rotational speed of the first and second pulley. The control system is capable of both clamping force control and transmission ratio control through determining the cylinder pressure for each of the pulleys. To this end the known control system is provided with a hydraulic circuit connecting the piston/cylinder assemblies to a pump and a reservoir for hydraulic medium and comprising two electronically controllable valves. The control system is furthermore provided an electronic control unit generating a respective control current for each valve in the hydraulic circuit at least based on a torque to be transmitted and on the rotational speed of the pulleys.
In the known control system the electronic control unit comprises two independently operating modules. A first module takes care of the clamping force control by setting the cylinder pressure in the piston/cylinder assembly of the first pulley through generating a first control current operating a first valve. Since the mechanical efficiency of the CVT decreases with increasing clamping force, the control system is usually arranged such that the cylinder pressure in the piston/cylinder assembly of the first pulley is maintained at a lowest possible level without belt slip occurring. Thereto the control system determines the minimum cylinder pressure in the first piston/cylinder assembly needed to prevent slip of the drive belt based on a torque to be transmitted by the transmission. In the known art the torque to be transmitted by the transmission is approximated on the safe side by adding to the actual torque level a value of 0.3 times the maximum possible torque level. Thus, the safety factor with which the actual torque level is multiplied to calculate the torque to be transmitted by the transmission is thus 1.3 at the maximum possible torque to be transmitted, but increases rapidly with a decreasing level of the torque to be transmitted. The technical effect of applying a safety factor being that belt slip is prevented in a manner and at a level practically satisfactory, even during abrupt and/or unpredicted changes in the torque to be transmitted. A second module takes care of the transmission ratio control by setting the cylinder pressure in the piston/cylinder assembly of the second pulley through generating a second control current operating a second valve.
It is an object of the invention to provide for a solution to the desire to improve the performance and efficiency of the CVT by lowering the safety factor while still effectively preventing slip of drive belt.
Although, in general the known control system provides a simple and stable CVT control, it was found that after prolonged use, drive belts can show an unexpected amount of wear having a slight adverse effect on the durability of the CVT. It is a further object of the present invention to identify the problem underlying unexpected wear of the drive belt and to provide for a solution, thereby improving the durability of the CVT.
It was found that the wear is due to the occurrence of mutual movement of drive belt and pulley discs, i.e. belt slip, which suggested that during operation of the CVT situations can occur wherein the clamping force control is unable to adequately prevent belt slip. So, even though in the known art a relatively large safety factor is applied to the value for the torque to be transmitted, belt slip still remains a problem. These practical findings suggest that it is not possible to lower the safety factor without compromising the durability of the drive belt.
From U.S. Pat. No. 5,707,314 a control system is known that is characterised by the feature that it determines a minimum cylinder pressure to prevent belt slip not only for the first pulley, but also for both for the first and the second pulley. In effect this means the clamping forces of both pulley are explicitly calculated. Based on these minimum cylinder pressures and on the ratio between the rational speeds of the first and second pulley, i.e. the transmission ratio, the control system then determines and controls both cylinder pressures to be equal, or higher than, the respective minimum cylinder pressure. With such system belt slip both on the first and second pulley may be prevent in most operational conditions.
However, according to the invention, it appeared that belt slip occurred at a high rate of change of the transmission ratio. It was furthermore found that the belt slip occurs between the pulley discs of a specific pulley and more in particular, in case of the CVT known from EP-A-0.451.887, the pulley discs of the second pulley, being the pulley used to control the transmission ratio.
According to the invention the control system and CVT having the below discussed features advantageously prevents the occurrence of belt slip and in doing so provides for a CVT with an improved durability and efficiency.
It is noted that the control system according to the invention determines both the cylinder pressure in the first and in the second piston cylinder assembly. It is common practice to use a feedback loop, wherein the actual value of a variable is fed back into a control system, when determining the variable. Therefore, it would appear that the control system according to the invention needs to be provided with two pressure sensors, one for each cylinder pressure. However, according to the invention, sufficient accuracy may be obtained, if only one cylinder pressure is measured using a pressure sensor and the other cylinder pressure is calculated from the measured cylinder pressure and the rotational speed of the pulleys as well as some transmission parameters. The cost of an additional pressure sensor thus being avoided.
Another advantage of the invention is that the approximation of the torque to be transmitted by the transmission may be made less safe compared to the known art without compromising the durability of the CVT. It was found that, when using the control system according to the invention, the torque to be transmitted by the transmission may be approximated by taking the level of the actual torque for each of the first and second pulley multiplied by a safety factor which is equal to 1.3 in a predominant part of a range of actual torque levels. The specific size of the part depends on the transmission ratio. In this manner a significant improvement of the overall efficiency of the transmission may be achieved.
According to the invention a cylinder pressure is maintained at a substantially higher level when a rate of change of the transmission ratio is relatively large than when the transmission ratio is constant. This may for example be effected by increasing the safety factor with an increasing rate of change of the transmission ratio. Since it was found that belt slip mainly occurs during a changing transmission ratio, the technical effect of this measure is that belt slip is prevented and that the performance and durability of the CVT is improved. According to the invention the relationship between the rate of change, RC, of the transmission ratio is given by the following equation:
  RC  =            -              M                  Nf          /          Ns                      ·    Ns    ·    Ff    ·          (              KsKf        -                  Fs          Ff                    )      
wherein:
MNf/Ns is an experimentally determined (positive) parameter which varies with the ratio of the rotational speeds of the pulleys (4, 5),
Ff is the force with which the drive belt (1) is clamped between the discs (8, 9) of the first pulley (4),
Fs is the force with which the drive belt (1) is clamped between the discs (10, 11) of the second pulley (5),
KsKf is the ratio of the forces Fs and Ff at which the rate of change would be zero.
From the equation it may be concluded that, if the transmission ratio defined as the rotational speed of the first pulley divided by the rotational speed of the second pulley increases, a desired rate of change of the transmission ratio may only be achieved at a certain minimum Fs and thus at a certain further minimum cylinder pressure in the piston/cylinder assembly of the second pulley. If the transmission ratio decreases, a desired rate of change of the transmission ratio may only be achieved at a certain minimum Ff and thus at a certain further minimum cylinder pressure in the piston/cylinder assembly of the first pulley. Thus, if the control system of the CVT is provided with an input signal representing a desired rate of change of the transmission ratio, a further minimum cylinder pressure may be determined either for the first or for the second pulley using the equation. Based on this further minimum cylinder pressure, on the minimum cylinder pressures and on the transmission ratio, the control system may then determine and control both cylinder pressures to be equal to, or higher than, the respective minimum and/or further minimum cylinder pressures. In this manner it is at all times possible to effectively prevent belt slip both between the pulley discs of the first and of the second pulley and to reliably achieve a desired rate of change of the transmission ratio.
According to a further development of the invention, the control system is provided with change rate restriction means that are capable restricting the rate of change of the transmission ratio in case the flow of hydraulic medium delivered by the pump is insufficient to maintain the desired cylinder pressures. The change rate restriction means may be able to determine the difference between the actual flow of hydraulic medium delivered by the pump and the flow required to achieve the further minimum cylinder pressure and thus to achieve the desired rate of change. The change rate restriction means are then arranged such that, as long as the difference is negative, the desired rate of change is reduced. Alternatively, the change rate restriction means may be able to determine whether the hydraulic system is capable of controlling an actual cylinder pressures to a desired level. If the change rate restriction means detect that the actual cylinder pressure does not reach the desired level, the desired rate of change is reduced. The change rate restriction means ensure that the hydraulic system is at all times capable of generating the further minimum cylinder pressure and belt slip is prevented even when the originally desired rate of change of the transmission ratio would yield belt slip.
As is known in the art, the response of the hydraulic system and of rotating parts of the transmission to a change in the control currents generated by the electronic control unit is often non-linear. Because of the non-linear response the control system is usually unable to accurately control the cylinder pressures to their respective desired level. This non-linear behaviour has an adverse effect on the accuracy with which the cylinder pressures may be controlled and therewith on the efficiency of the transmission. It is, for instance, not feasible to use the integrating action of a PID-regulator due to the non-linear behaviour. Fast changes of the transmission ratio are difficult or even impossible to achieve reliably, because for in this case relatively large differences between the cylinder pressure of the first and of the second pulley are needed, in which case the non-linear behaviour is even more pronounced. This is especially important if a relatively small safety factor is adopted, as is allowed by control system according to the invention. It is therefore advantageous for the control system to be able to account for the non-linear response of the transmission. To this end the control system may be provided with linearisation module containing a mathematical representation of a mass balance of the hydraulic system. Such a mass balance may be construed in a known manner from the flow of hydraulic medium circulated by the pump, the flow characteristic of the valves, and the flow to and from the piston/cylinder assemblies when the transmission ratio changes. Incorporating leakage flows and a value for the compressibility of the hydraulic medium in the mass balance can improve the accuracy of the control system even further. The mathematical representation of the mass balance is incorporated in the linearisation module of the control system such that the system can determine one or more control current required for effecting a desired pressure response in the hydraulic circuit. In a further development of the invention the mathematical representation is contained in a sub-unit which is physically separable from the electronic control unit. This allows the control system to be easily adopted to several layouts of the hydraulic system by exchanging the sub-unit.
According to the invention it is preferable to use a mathematical representation of the hydraulic circuit based on flows of hydraulic medium, because the flows may be derived directly from known transmission variables such as the rotational speed of a pulley drivingly associated with the pump and the rate of change of the transmission ratio, which determines the flows to and from the piston/cylinder assemblies of the pulleys.
According to a further elaboration of the invention the electronic control unit may comprise separate control modules for clamping force control and for transmission ratio control. The first control module controls the clamping force and is capable of generating a first control current for operating the first valve and the second control module controls the transmission ratio and is capable of generating a second control current for operating the second valve. This set-up of the electronic control unit is particularly suited for a hydraulic system wherein the first valve is a pressure control valve which determines the cylinder pressure of the first pulley and the second valve is a flow control valve which controls a flow of hydraulic medium to and from the piston/cylinder assembly of the second pulley. This type of hydraulic system allows the cylinder pressure of the second pulley to become only as high as the cylinder pressure of the first pulley. However, the design of the piston/cylinder assemblies is such that clamping force of the first pulley can become higher than that of the second pulley. The second control module incorporates both a PI-regulator and the linearisation module, so that the control of the cylinder pressure of the second pulley is more accurate and the transmission ratio is controllable with a high degree of accuracy. The first control module determines the minimum cylinder pressures and the further minimum cylinder pressure, as well as a desired ratio of the first and second cylinder pressures, based on torque signal representing a torque to be transmitted by the transmission multiplied by a safety factor, signals representing the rotational speed of the pulleys and a signal representing a desired rate of change of the transmission ratio. Subsequently, the first control module selects the cylinder pressure for the first pulley to be the highest of a cylinder pressure:
given by the minimum cylinder pressure in the piston/cylinder assembly of the first pulley, or
given by the desired ratio of the first and second cylinder pressures and a minimum cylinder pressure in the piston/cylinder assembly of the second pulley, or
given by the further minimum cylinder pressure in the piston/cylinder assembly of the first pulley, or
given by the desired ratio of the first and second cylinder pressures and a further minimum cylinder pressure in the piston/cylinder assembly of the second pulley.
Based on a pressure difference signal representing the difference between the highest cylinder pressure of the first pulley and the actual cylinder pressure of the first pulley measured by means of a pressure sensor and on the valve pressure/current characteristic, an appropriate control current for operating the first valve is generated by the first module. The second control module determines the transmission ratio by controlling the rotational speed of the second pulley based on a speed difference signal representing the difference between a measured rotational speed of the second pulley and a desired rotational speed of the second pulley which is calculated from the measured rotational speed of the first pulley and the desired transmission ratio. The first and second control modules are capable of mutually providing each other with a signal at least representing the cylinder pressure in the first piston cylinder assembly and the desired rotational speed of the second pulley. This elaboration of the invention has the advantage, that the control system may be used in combination with the so called Master/Slave hydraulic layout which is widely used in contemporary CVTs and provides for a relatively simple and cost effective transmission control.
According to the invention the CVT is suited for application in a motor vehicle having an engine with an engine shaft and a drive shaft for driving driven wheels of the vehicle, in which case it is preferred that the first pulley is drivingly connected to the first pulley and the engine shaft is drivingly connected to the second pulley. The control system and CVT according to the invention are particularly suited for use in combination with an adjustable clutch applied in the torque path between engine and driven wheels, which clutch is adjusted to be capable of transmitting a maximum amount of torque, the maximum amount of torque being smaller than a torque transmittable by the continuously variable transmission without relative movement between the drive belt and the pulley discs. The technical effect of such a combination of adjustable clutch and CVT according to the invention being that belt slip virtually eliminated and that a small safety factor may be applied improving the fuel efficiency of the motor vehicle.